The present invention relates to semiconductor devices having liquid crystal driving circuitry and also to testing methods thereof. More particularly, but not exclusively, this invention relates to useful techniques for application to a liquid crystal driving circuit which selects a predetermined level of voltage based on data as accommodated in a storage unit and then outputs it to a respective one of multiple external terminals.
The technologies that the present inventors have studied encompass those relating to liquid crystal driving circuits such as generally used color thin-film transistor (TFT) drivers for mobile use, one of which is configured as shown in FIG. 11, for example. This liquid crystal driver circuit is operable to hold the data which are written into a display data storage random access memory (RAM) 12 through an external interface in a line buffer 31 in units of lines of liquid crystal display data and then select, in each switch circuit 34 within a gradation voltage selecting circuit 33, a gradation or gray-scale voltage with a predetermined level generated at a gradation voltage generating circuit 32 on the basis of the liquid crystal display data being held in the line buffer 31 to thereby output it to each output terminal. And, in responding to a gradation/gray-scale voltage generated by this liquid crystal driver circuit, each picture element or “pixel” of a liquid crystal display (LCD) panel is electrically charged up to a hold capacitance amount whereby the brightness or luminance of each pixel is controlled on the LCD panel side.
At the time of testing this liquid crystal driver circuit, it is arranged to perform several operations which follow. Apply an arbitrary test pattern to the liquid crystal driver circuit from a tester 35 through an external interface. Then, write data into the display data RAM 12 and execute control of a display controller 11, thereby causing a given gradation voltage to output toward an output terminal from each switch circuit 34 within the gradation voltage selector circuit 33. This output voltage is measured by the tester 35 to thereby perform the test required.
As explained above, the liquid crystal driver circuit is such that a digital functional unit which is comprised of the display controller and the display data RAM and an analog functional unit made up of the gradation voltage generator circuit and gradation voltage selector circuit operate together in an integral or united way. Accordingly, in the case of implementation of digital functional tests of the liquid crystal driver circuit, a need is felt to measure a prespecified potential level of gradation voltage to be output from the output terminal. The liquid crystal driver circuit is faced with problems which follow: it is difficult to increase the driving ability or “drivability” of any gradation voltage output for the purpose of lowering power consumption and, for this reason, it is impossible to realize speed-up or acceleration of a gradation voltage measurement; on the other hand, due to an increase in number of test items in accordance with the quest for higher performances, the test time increases so that it becomes difficult to reduce costs.
Additionally in the above-noted liquid crystal driver circuit, the one such as shown in FIG. 12 is considered, which is constituted from a gradation voltage generator circuit 32 and a gradation voltage selector circuit 33 (switch circuits 34). In this gradation voltage generator circuit 32, a gradation or gray-scale voltage with any given n tone levels is generated by potentially dividing a gradation generation voltage V0 into n portions at a given rate, while using the voltage V0 as a reference. And, in each switch circuit 34 which is disposed within the gradation voltage selector circuit 33, a given gradation voltage is selected and output in a way pursuant to the gradation setup data being presently held in the line buffer.
In this liquid crystal driver circuit, when performing testing of the gradation voltage at output terminals, use the gradation setup data being set in the line buffer to set an output voltage of each output terminal at a prespecified gradation voltage value; then, perform voltage measurement by using an analog-to-digital (AD) converter or the like on a per-output terminal basis. This is measured with respect to all the gradation voltages to thereby perform the test. Accordingly, the prior known approach has the following problems to be solved: it is difficult to shorten the length of a test time period and speed up the test due to the presence of a limitation to the above-noted gradation output voltage drivability; and, the test time increases with an increase in number of output terminals of the liquid crystal driver circuit in a way corresponding to a growth in high precision of LCD panels or alternatively an increase in number of gradation or tone levels, resulting in difficulty of cost reduction.
In order to solve these problems, a technique for acceleration of the test has been proposed, which is disclosed for example in JP-A-2002-197899. This technique aims at shortening of the test time by employing an arrangement in which the liquid crystal driver circuit performs a gradation test while retaining liquid crystal display data in a storage circuit such as a line buffer through the display data RAM and, at the same time, interrupts writing into the line buffer to thereby perform testing of the display data RAM.